1. Technical Field
The subject invention refers to a two-stroke crankcase scavenged internal combustion engine in which one or more piston ported air passages are arranged between one or more air inlets and the upper part or ends of one or more corresponding transfer or scavenging ducts. Fresh air is added at the top of the transfer ducts and is intended to serve as a buffer against the air and fuel mixture located therebelow. This buffer is mainly lost through the exhaust outlet during the scavenging process; the stage after combustion during which the exhaust gases are purged from the cylinder chamber. It is in this manner that the fuel consumption and the exhaust emissions associated with the instant type of two-stroke internal combustion engine design are reduced; that is, there is a reduced amount of uncombusted hydrocarbons exhausted to the atmosphere. That means that there is a more complete utilization of all fuel supplied to the engine, and less unburned and polluting fuel is released to the atmosphere. Engines configured according to the teachings of the present disclosure are particularly appropriate for utilization in handheld working tools because of their high power-to-mass and high power-to-package size characteristics, as well in other suitable applications.
2. Background of the Invention
Two-stroke internal combustion engines of the above mentioned scavenged type are generally known. They beneficially reduce fuel consumption and exhaust emissions; but negatively, the air-to-fuel ratio in such an engine is difficult to control.
U.S. Pat. No. 5,425,346 shows an engine with a somewhat different design than that which is described above. In the design of the '346 patent, channels are arranged in the piston of the engine, which at specific piston positions align with ducts arranged in the cylinder. Fresh air, as shown in FIG. 7, or exhaust gases can thereby be added to the upper part of the transfer ducts. This only happens at the specific and discrete piston positions where the ducts in the piston and the cylinder are aligned, which is also referred to as coming into registration. In fact, registration occurs at two broken apart points in time; one point when the piston moves downwards and a second point when the piston moves upwards. Each, however, occur when the piston is far away from a top dead center position. To avoid unwanted flow in the wrong direction in the latter case, check valves are arranged at the inlet to the upper part of the transfer ducts. The amount of fresh air that can be added is therefore limited because of the short time period that air is supplied and because the presence of the necessary check valve(s) causes substantial resistance to flow when it does occur. These types of check valves, usually referred to as reed valves, however, also have a number of other disadvantages. Such valves frequently have a tendency to come into resonant oscillations causing operational difficulties at high rotational engine speeds which two-stroke internal combustion engines often reach. Still further, the added component of the reed valve adds to the total cost of the engine, as well as increases the number of constituent engine components further complicating the design. In the operation of an engine such as that disclosed in the '346 patent, the amount of fresh air added to the engine is manipulated through the use of a variable inlet; that is, an inlet that can be advanced or retarded in the work cycle. This is, however, is an unnecessarily complicated solution.
International Patent Application PCT/JP98/02478 published under the number WO98/57053 shows a few different embodiments of an engine in which air is supplied to transfer ducts via L-shaped or T-shaped recesses in the piston. Thus, there are no check valves. In all embodiments, however, these piston recesses have, where they meet a respective transfer duct, a very limited height that is essentially equal to the height of the actual transfer port. A consequence of this design is that the passage for air delivery through the piston to the transfer port is opened significantly later than the opening of the passage for the air/fuel mixture to the crankcase by the piston. The period for the air supply is consequently significantly shorter than the period for the supply of air/fuel mixture, where the period can be quantified based on crank angle or time. This can complicate the control of the total air-to-fuel ratio of the engine. This also means that the amount of air that can be delivered to the transfer duct is significantly limited because the underpressure condition utilized to drive this additional air has decreased significantly since the inlet port has already been open during a certain period of time when the air supply is opened. This implies that both the period and the driving force for the air supply are small.
Furthermore, the flow resistance in such L-shaped and T-shaped ducts is relatively high. In general this high resistance to fluid flow therethrough can be attributed to the sharp bend(s) or turns created by the L- and T-shapes, and in the instantly described case, additionally because the cross section of the duct is small close to the transfer port. Regarding the fluid flow associated with these passages, just after the air initially enters the passage, it is forced to change direction abruptly away from a lateral direction with respect to the cylinder to a portion of the passage that is oriented outwards and then immediately downwards through two successive bends or curves, each measuring 90°, in rapid succession. This is due to the fact that the transfer ducts of the engine are running in a radial and vertical direction to the cylinder. In all, this contributes to increasing the flow resistance and to reducing the amount of air that can be delivered to the transfer ducts. A consequence of such a design is that it inhibits the possibilities for reducing the engine's fuel consumption and exhaust emissions.